1. Field of the Invention The present invention generally relates to a compact Base Station (BS). More particularly, the present invention relates to an apparatus and method for setting the transmit power of a compact BS in a broadband wireless communication system.
2. Description of the Related Art
A BS in a cellular broadband wireless communication system communicates with Mobile Stations (MSs) within its cell area via radio channels. The statuses of the radio channels vary with the movement of the MSs. If an MS is located in a physically shielded area, for example, in a propagation shadow area such as an office or a house, the channel status between the MS and a BS is very poor, thus making it difficult to establish and maintain reliable communications between them. Installation of a compact BS serving as a BS in a shadow area such as an office or a house is under consideration as a way to address this problem.
A compact BS is useful in the following two exemplary cases. The first exemplary case is where users cannot receive a service from an outdoor macro BS installed by a service provider due to signal attenuation caused by the walls of a building or obstacles. The second case is where a user installs a personal BS within a house or an office in order to reduce costs.
Compared to a high-power BS installed by the service provider, the compact BS should adaptively control its transmit power according to the propagation environment of its location. A constraint on the transmit power of the compact BS is that an acceptable data rate should be provided to its users, while minimizing effects on the legacy system. In addition, handover to and from an external BS should be available within an allowed transmit power of the compact BS.
A Code Division Multiple Access (CDMA) compact BS for home use, referred to as a Ubicell, adopts a power ranging scheme that increases its transmit power until a foreign user is detected. Device Identifiers (IDs) of users such as Media Access Control (MAC) IDs or Electronic Serial Numbers (ESNs) are registered to the compact BS and the compact BS controls its output level by increasing its transmit power until an initial access or a handover from an unregistered user is detected.
This conventional technology is based on the premise that the compact BS has full knowledge of its registered users. If the compact BS is installed for home use, its implementation is not challenging because there would only be a small number of registered users. On the other hand, in a large building, the compact BS is harder to implement due to there being a large number of registered users. What is worse, when a foreign user moves close to the building, the compact BS should operate without changing its transmit power and should consider the movement of the foreign user to be an exceptional situation. That is, the compact BS should determine whether an access from a foreign user has been triggered by his movement into the area serviced by the compact BS or a result of an excessive transmit power of the compact BS.
As described above, registration of users to the compact BS is required in order to control the transmit power of the compact BS based upon a detection of an access (or handover) of a foreign user, and the inability to accurately determine whether the foreign user has moved into the area serviced by the compact BS. As a consequence, it is difficult to implement the compact BS.